Potentiometric Study of the Flavins

The yellow dyestuff which, in the form of its phosphate ester and combined with a particular protein, forms Warburg’s yellow respiration ferment (1) is a derivative of isoalloxazine (Formula I, with R = H). This was first suggested by Stern and Holiday (2) and definitely proved by Kuhn it al. (3, 4) and Karrer et al. (5). This dyestuff has been identified by GyBrgy, Kuhn, and Wagner-Jauregg (3) with vitamin Bz, or better, with one of those substances which were formerly, as a group, designated as vitamin Bz. The vitamin is 6,7-dimethyl-Y-d-riboisoalloxazine. Isoalloxazine itself is tautomeric with alloxazine in which the H in position (9) is shifted to position (l), and seems to exist to the greater extent in the latter form. In bhe substituted compounds no ambiguity exists, so we shall omit the prefix iso. The present studies are concerned with alloxazine, g-methyl alloxazine, 9-dglucoalloxazine, and with four 6,7-dimethyl derivatives, which will be designated as flavins, namely g-methyl flavin (lumiflavin), 9-d-glucoflavin, 9-Z-araboflavin, and g-d-riboflavin (vitamin BP). All these dyes were prepared synthetically according to Kuhn’s method. The similarity of the structure of these compounds to those of the phenazine group, especially to pyocyanine, is obvious. Accordingly, they share with these dyes t,he ability of being reversibly reduced and the ability to form, on partial reduction in acid solution, an intermediate form (6), referred to as the X form, half-way bet.ween the reduced, or R form, and the totally oxidized, or T form. Kuhn and Wagner-Jauregg (7) were the first to show experimentally this analogy of lactoflavin and pyocyanine. Any alloxazine or flavin dye, dissolved in strong HCl, passes on reduc-